You can stop now... it must be prime! You don't need to keep checking for divisibility by the next primes (
13
,
17
,
19
,
23
,
etc.). The reason is that if
13
went in evenly, then we would have
127
=
13
×
n
for some number
n
. But then
n
would have to be less than
13
... and we already know that
127
is not divisible by any number smaller than
13
.